ENH: only print errors if QT_REQUIRED is set

Modules/FindQt.cmake

@@ -126,7 +126,9 @@ ELSE(NOT QT3_INSTALLED AND NOT QT4_INSTALLED)
     ENDIF(QT_REQUIRED)
   ENDIF(NOT QT_FOUND AND NOT DESIRED_QT_VERSION)
   IF(NOT QT_FOUND AND DESIRED_QT_VERSION)
-    MESSAGE(SEND_ERROR "CMake was unable to find QT version: ${DESIRED_QT_VERSION}. Set advanced values QT_QMAKE_EXECUTABLE and QT${DESIRED_QT_VERSION}_QGLOBAL_FILE.")
+    IF(QT_REQUIRED)
+      MESSAGE(SEND_ERROR "CMake was unable to find QT version: ${DESIRED_QT_VERSION}. Set advanced values QT_QMAKE_EXECUTABLE and QT${DESIRED_QT_VERSION}_QGLOBAL_FILE.")
+    ENDIF(QT_REQUIRED)
   ENDIF(NOT QT_FOUND AND DESIRED_QT_VERSION)
 ENDIF(NOT QT3_INSTALLED AND NOT QT4_INSTALLED)
 MARK_AS_ADVANCED(QT3_QGLOBAL_H_FILE QT4_QGLOBAL_H_FILE QT_QMAKE_EXECUTABLE)